Recently, radio communication systems have been in widespread use for executing wireless communication by cell phones (user terminals). In radio communication systems, usually, voice communication is carried out through a circuit-switching network, whereas data communication is carried out through a packet switching network (for example, see Patent Document 1).
On the other hand, a technology is proposed for executing both voice communication and data communication (non-voice data communication) through a single packet switching network (IP network). In this technology, not only normal data (non-voice data), but also voice data (data indicating voice signals) is transformed into packets and is then transmitted.